UV curable inks (flexo, offset, screen and inkjet) that are used for food packaging applications must fulfill very low migration level. Part of migration originates from the photoinitiator and from products of photo cleavage. Benzophenone is the most widely used photoinitiator for ultraviolet (UV) cured overprint varnishes as it has good surface curing, low yellowing and good solubility and is cheap and widely available. However, benzophenone is also known for its relatively strong odor and its ability to migrate and get extracted from print into foodstuffs, even through packaging such as board and plastic wrappers (see EP 1 438 282 B1). This has been partially solved by using “polymeric photoinitiators”, i.e. photoinitiators with higher molecular weight. Most of the polymeric photoinitiators have solubility issues, poor reactivity and have a big impact on ink flow. When used in higher concentration to increase reactivity, they often act as plasticizer which is detrimental for the mechanical properties of the cured ink.
There is thus a need to develop photo-reactive binders with no or low migration level along with a high UV reactivity and low detrimental effect on ink flow and mechanical properties of the cured ink even when it is used in large concentration.